At present, substrates of LED blue light epitaxial wafers are mainly sapphire. The LED technology based on a sapphire substrate has two serious problems. Firstly, the mismatch rate of the sapphire and a GaN crystal lattice is as high as 17%. Such high lattice mismatch results in a high defect density of the sapphire based LED epitaxial wafer, thereby greatly influencing the luminous efficiency of an LED chip. Secondly, the price of the sapphire substrate is very high, resulting in very high production cost of a nitride LED.
Another major reason for the low luminous efficiency of the LED chip is that the current GaN based LEDs in wide use have polarity. At present, the most ideal material for manufacturing of a high efficient LED device is GaN. The GaN is of a closely packed hexagonal crystal structure, whose crystal faces are divided into a polar face, which is a c face [(0001) face] and non-polar faces, which include an a face [(11-20) face] and an m face [(1-100) face]. Currently, most GaN based LEDs are constructed based on the polar face of the GaN. On the polar face of the GaN, the centroid of the Ga atom cluster does not coincide with the centroid of the N atom cluster, thus forming an electric dipole and generating a spontaneous polarization field and a piezoelectric polarization field, and further, causing a Quantum-confined Stark effect (QCSE). The QCSE causes electrons being separated from holes, and reduces the radiation recombination efficiency of charge carriers, which in turn influences the luminous efficiency of LED, and causes instability of a light emitting wavelength of the LED.